Caliciviruses are reported to be an important cause of illness in cats. A wide variety of symptoms are observed such as fever, rhinitis, sneezing, mild conjunctivitis, ocular discharge, vesicles in the external nares, oral mucosa or on the tongue, pneumonia, tracheal bronchitis, diarrhea, muscle soreness, stiff gate, and hyperesthesia. Opportunistic bacterial infections often accompany FCV infections, which complicate treatment and recovery. Severe FCV infections may lead to death, especially in juvenile cats. It should be noted that such signs, although reportedly common in natural cases, are not always prominent in experimental infections. It would appear that various field strains of feline calicivirus (FCV) either differ in their disease-causing potential, or that concurrent infection with other agents influences the disease symptoms.
Vaccines against feline calicivirus have been available for more than two decades. Although numerous FCV serotypes exist, certain strains, such as F9, were found to induce antibodies against a broad range of FCV strains. See J. L. Bittle & W. J. Rubic, Am. J. Vet. Res. 37:275-78 (1976). As a result, the earliest vaccines against feline calicivirus employed a modified or attenuated version of the FCV-F9 strain. See U.S. Pat. No. 3,937,812 to J. L. Bittle & W. J. Rubic, which is herein incorporated by reference in its entirety.
While the vaccines from FCV-F9 and other commercially available vaccines provide protection from many field isolates, it is not true that these vaccines prevent infection from all strains. Moreover, as FCV continues to evolve, FCV-F9 based vaccine provides protection against fewer and fewer field isolates (Lauritzen et al., 1997, Vet Microbiology, 56:55-63). In addition, veterinary practitioners have expressed concerns over the efficacy of vaccines based on a single serotype. Indeed, field studies suggest that vaccines derived from the FCV-F9 strain provide insufficient immunity against many strains of feline calicivirus. See, e.g., N. C. Pederson et al., Feline Pract. 13(1):26-35 (1983); S. Dawson et al., Vet. Rec. 132:346-50 (1993). Practitioners have also raised concerns about the administration of a modified live virus that may, in some circumstances, cause disease in otherwise healthy animals. Researchers have reported that inadvertent oral dosing of a subcutaneously-administered FCV vaccine resulted in acute disease. See R. C. Povey, Feline Pract. 7(5):12-16 (1977). There is, therefore, continued interest in developing a vaccine, which by itself or in combination with other vaccines, would provide the desired protection upon vaccination of a cat. We describe several isolates here which have been isolated from cats and provide a means of providing broad protection in immunized cats.